14,555 research outputs found
Black-hole horizon and metric singularity at the brane separating two sliding superfluids
An analog of black hole can be realized in the low-temperature laboratory.
The horizon can be constructed for the `relativistic' ripplons (surface waves)
living on the brane. The brane is represented by the interface between two
superfluid liquids, 3He-A and 3He-B, sliding along each other without friction.
Similar experimental arrangement has been recently used for the observation and
investigation of the Kelvin-Helmholtz type of instability in superfluids
(cond-mat/0111343). The shear-flow instability in superfluids is characterized
by two critical velocities. The lowest threshold measured in recent experiments
(cond-mat/0111343) corresponds to appearance of the ergoregion for ripplons. In
the modified geometry this will give rise to the black-hole event horizon in
the effective metric experienced by ripplons. In the region behind the horizon,
the brane vacuum is unstable due to interaction with the higher-dimensional
world of bulk superfluids. The time of the development of instability can be
made very long at low temperature. This will allow us to reach and investigate
the second critical velocity -- the proper Kelvin-Helmholtz instability
threshold. The latter corresponds to the singularity inside the black hole,
where the determinant of the effective metric becomes infinite.Comment: LaTeX file, 12 pages, 3 Figures, version accepted in JETP Letter
Superfluid analogies of cosmological phenomena
Superfluid 3He-A gives example of how chirality, Weyl fermions, gauge fields
and gravity appear in low energy corner together with corresponding symmetries,
including Lorentz symmetry and local SU(N). This supports idea that quantum
field theory (Standard Model or GUT) is effective theory describing low-energy
phenomena. * Momentum space topology of fermionic vacuum provides topological
stability of universality class of systems, where above properties appear. *
BCS scheme for 3He-A incorporates both ``relativistic'' infrared regime and
ultraviolet ``transplanckian'' range: subtle issues of cut-off in quantum field
theory and anomalies can be resolved on physical grounds. This allows to
separate ``renormalizable'' terms in action, treated by effective theory, from
those obtained only in ``transPlanckian'' physics. * Energy density of
superfluid vacuum within effective theory is ~ E_{Planck}^4. Stability analysis
of ground state beyond effective theory leads to exact nullification of vacuum
energy: equilibrium vacuum is not gravitating. In nonequilibrium, vacuum energy
is of order energy density of matter. * 3He-A provides experimental prove for
anomalous nucleation of fermionic charge according to Adler-Bell-Jackiw. *
Helical instability in 3He-A is described by the same equations as formation of
magnetic field by right electrons in Joyce-Shaposhnikov scenario. * Macroscopic
parity violating effect and angular momentum paradox are both desribed by axial
gravitational Chern-Simons action. * High energy dispersion of quasiparticle
spectrum allow to treat problems of vacuum in presence of event horizon, etc.Comment: draft of review for Physics Reports, RevTex file, 113 pages, 26
figures; new sections and references are adde
Contingency Model Predictive Control for Automated Vehicles
We present Contingency Model Predictive Control (CMPC), a novel and
implementable control framework which tracks a desired path while
simultaneously maintaining a contingency plan -- an alternate trajectory to
avert an identified potential emergency. In this way, CMPC anticipates events
that might take place, instead of reacting when emergencies occur. We
accomplish this by adding an additional prediction horizon in parallel to the
classical receding MPC horizon. The contingency horizon is constrained to
maintain a feasible avoidance solution; as such, CMPC is selectively robust to
this emergency while tracking the desired path as closely as possible. After
defining the framework mathematically, we demonstrate its effectiveness
experimentally by comparing its performance to a state-of-the-art deterministic
MPC. The controllers drive an automated research platform through a left-hand
turn which may be covered by ice. Contingency MPC prepares for the potential
loss of friction by purposefully and intuitively deviating from the prescribed
path to approach the turn more conservatively; this deviation significantly
mitigates the consequence of encountering ice.Comment: American Control Conference, July 2019; 6 page
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